Recently, concerns over environmental conservation have lead to considerable research and development in the field of clean energy. Amongst this research, solar cells are attracting much attention due to the limitless nature of the sunlight that acts as the power source, and the fact that solar cells are non-polluting. Conventionally, electric power generation from sunlight using solar cells has employed bulk solar cells, which are prepared by producing bulk crystals of single crystal silicon or polycrystalline, and then slicing these crystals to generate thick sheet-like semiconductors. However, large amounts of time and energy are required to grow the silicon crystals used in these bulk solar cells, and the subsequent production process also requires complex steps. As a result, improving the mass productivity is difficult, and providing low-cost solar cells has proven problematic.
On the other hand, in the case of thin-film semiconductor solar cells (hereafter referred to as “thin-film solar cells”) that use a semiconductor such as an amorphous silicon having a thickness of not more than several micrometers, a semiconductor layer of the desired thickness that acts as the photovoltaic layer need simply be formed on an inexpensive substrate of glass or stainless steel or the like. Accordingly, because these thin-film solar cells are thin and lightweight, are cheap to produce, and can be easily produced as large surface area cells, it is thought that they will be the predominant form of solar cells in the future.
Thin-film solar cells are classified as superstrate cells or substrate cells depending on their structure. In a superstrate solar cell, in which the incident light enters the cell from the side of the transparent substrate, the cell typically has a structure in which the substrate, a transparent electrode, a photovoltaic layer, and a back electrode are formed in sequence. In contrast, in a substrate solar cell, a structure is adopted in which the substrate, a back electrode, a photovoltaic layer, and a transparent electrode are formed in sequence. In a superstrate solar cell in which the photovoltaic layer is formed of a silicon-based material, investigations have been conducted into enhancing the electric power generation efficiency by adopting a structure in which a transparent electrode, an amorphous silicon, a polycrystalline silicon, and a back electrode are formed in sequence (for example, see Non-Patent Document 1). In the structure disclosed in Non-Patent Document 1, the amorphous silicon and the polycrystalline silicon constitute the photovoltaic layer.
Particularly in those cases where the photovoltaic layer is formed of a silicon-based material, because the absorption coefficient for the photovoltaic layer is comparatively small, if the photovoltaic layer is formed with a thickness in the order of several micrometers, then a portion of the incident light passes through the photovoltaic layer, and this transmitted light is unable to contribute to electric power generation. Accordingly, generally, either the back electrode is formed as a reflective film, or a reflective film is formed on top of the back electrode, thereby reflecting the light that is transmitted through the photovoltaic layer without undergoing absorption and returning the light to the photovoltaic layer for a second time, thus increasing the electric power generation efficiency.
In previous thin-film solar cell development, the electrode and the reflective film have been formed using a vacuum deposition method such as sputtering or the like. However, because maintaining and operating a large-scale vacuum deposition apparatus requires enormous cost, it is anticipated that lower cost production processes using wet film formation methods will be developed.
As an example of a conductive reflective film formed using a wet film formation method, a method has been disclosed in which a reflective film formed on the back side of the photovoltaic element is formed using an electroless plating method (for example, see Patent Document 1).
Further, a simpler method is also being investigated in which a metal having a high reflectance such as silver is converted to nanoparticles, and these nanoparticles are then applied by a coating method (for example, see Patent Document 2). Furthermore, a method that involves the application of metal nanoparticles has also been disclosed as a method of forming a metal coating having high metallic gloss similar to a plating layer on the surface of a substrate (for example, see Patent Document 3). Moreover, in a superstrate solar cell that employs a substrate-transparent electrode-photovoltaic layer-transparent electrode-back reflective electrode type structure, one example of a method of forming the transparent electrode positioned adjacent to the substrate includes applying a coating liquid containing microparticles of a conductive oxide dispersed within a dispersion medium, and then using a heating method to dry and cure the coating liquid (for example, see Patent Document 4).
However, the electroless plating method disclosed in the aforementioned Patent Document 1 includes steps of forming a plating protective film on the surface, subsequently treating the surface to undergo the plating treatment with a hydrofluoric acid solution, and then dipping the structure in an electroless plating solution, meaning not only is the method complex, but the generation of considerable amounts of waste liquids can be expected.
In the methods of Patent Documents 2 and 3 that involved the application of metal nanoparticles, the reflectance when evaluated from the substrate side of the reflective film applied to the substrate surface (hereafter referred to as “the back side reflectance”) tends to be inferior to the reflectance on the exposed surface on the opposite side (hereafter referred to as “the surface reflectance”). It is thought that this difference is because holes are formed between the reflective film and the substrate, and light undergoes repeated reflection within these holes, resulting in a reduction in the reflectance. Further, in those cases where reflected light that has passed through a hole is irradiated onto the substrate at an incident angle that is larger than the critical angle, the reflected light undergoes total reflection at the interface between the hole and the substrate, and it is thought that the reflectance decreases as a result of an increase in this type of total reflection. Furthermore, in the metal nanoparticle coating methods disclosed in Patent Documents 2 and 3, only the conductive reflective film is formed by a coating method, and neither method represents a process in which both the conductive reflective film and the transparent conductive film are formed by a coating method.
In the aforementioned Patent Document 4, the wet deposition method is used only for the transparent conductive film positioned adjacent to the substrate in the superstrate solar cell, and the transparent conductive film positioned adjacent to the back reflective, electrode is formed by a sputtering method that represents a conventionally employed vacuum process.
Non-Patent Document 1: Shozo Yanagida et al., “The leading edge of thin-film solar cell development—towards higher efficiency, mass productivity, and more widespread use”, NTS Co., Ltd., March 2005, page 113, FIG. 1(a)
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 05-95127 (claim 1, paragraph [0015])
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. Hei 09-246577 (paragraph [0035])
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2000-239853 (claim 3, paragraph [0009])
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. Hei 10-12059 (paragraphs [0028] and [0029])